FIG. 1 is a schematic view showing a conventional wavelength tunable laser apparatus of the type mentioned, for example, in "Applied Optics", July 1974, vol. 13, No. 7, pp. 1625-1628. In the figure, the reference numeral 1 denotes a laser oscillator which is, in this case, a dye laser. The numeral 2 denotes a partially reflecting mirror, 3 a Fabry-Perot etalon (hereinafter abbreviated as "FP") provided with a gap, 4 a closed vessel in which the FP 3 is hermetically sealed with a gas, 5 a laser beam, 6 a pressure gauge for measuring the level of gas pressure inside the closed vessel 4, 7 and, 8 valves, and 9 a grating.
The operation will next be explained. The laser beam emitted from the laser oscillator 1 has a wavelength selected by various elements incorporated in the oscillator. In this prior art, the range of wavelengths is narrowed by means of spectroscopic elements, that is, the grating 9 and the FP 3, which are inserted in the resonator. Moreover, by adjusting these spectroscopic elements, it is possible to set the wavelength at a desired one within the range of oscillation wavelengths that have been present from the outset.
In this prior art, wavelength selection is effected by changing the angle of inclination of the grating 9 and further changing the gas pressure inside the closed vessel 4 to thereby vary the refractive index of the gas in the gap provided in the FP 3. Coarse adjustment of the wavelength can be effected by changing the angle of inclination of the grating 9, and fine adjustment of the wavelength can be achieved by adjusting the gas pressure in the gap of the FP 3. The adjustment of the gas pressure is effected by measuring the level of pressure with the pressure gauge 6 and opening or closing the valves 7 and 8.
The conventional laser apparatus which is arranged in the manner described above has the problem that it is necessary to use a pressure gauge which has a degree of accuracy of, for example, better than 0.1% of the full scale in order to perform wavelength tuning with high precision, e.g., tolerances of .+-.0.001 nm.